I don't even know why we are limited to 10 times anymore. As the game is saved to HDD, space isn't a problem unlike the bad old days of the memory cards.
I think the fastest times board should only show the fastest time of that car, so the only way you can have multiple instances of the say Honda Civic Type R 98, is if you have actually got two, and have driven them both around the track to get a time.
So if you do a new faster time in a car it just overwrites the existing fast time for that car, and if it is faster than a other cars in the table, it re-jigs the table to move the faster car up, and the slower cars down the positions.